restore support for extending babel configuration

[thescientist13]

Related Issue

#532

Summary of Changes

1. Restore Babel configuration extend spec
2. Set babelHelpers to bundled since I got this error and also it seems like the best option anyway 🤷‍♂️

Error: You have declared using "runtime" babelHelpers, but transforming /Users/owenbuckley/Desktop/greenwood/packages/plugin-babel/test/cases/options.extend-config/src/scripts/main.js resulted in "inline". Please check your configuration.

---

Tried to add ESM config support, e.g

module.exports = {
  plugins: [
    '@babel/plugin-proposal-class-properties',
    '@babel/plugin-proposal-private-methods'
  ]
};

But it ended up leading to a cascade of issues because

1. When running the spec, was getting this error because... Babel already seems to support ESM

    Parsing error: /path/to/.babelrc.mjs: Error while loading config - You appear to be using a native ECMAScript module configuration file, which is only supported when running Babel asynchronously

2. In order to import an ESM config, you need to be able to use await
3. Which means getConfig would have to be async
4. Which means all usages of getting a provider would need to async
5. Which means everywhere we are doing "faux" `aysnc, would need to be refactored

So given all that, plan will be to add these to the tracking items in #815

1. Add Babel as tool to support ESM config
2. Make an issue to make Greenwood "explicitly" async through and through, and use more for ... in and less reduce 😵